In 1990, epidemic diphtheria began to emerge in the New Independent States of the former Soviet Union; by 1996, approximately 125,000 cases of diphtheria and 4,000 deaths had been reported. Disease outbreaks are yet another stimulus to discovery. Certainly no one wishes for large numbers of people to become ill; however, outbreaks usually are investigated in considerable detail, whereas single cases are much less likely to receive such attention. Certainly this rule of thumb held true with the Legionnaires' disease outbreak in Philadelphia in 1976, the toxic shock syndrome outbreak several years later, and the outbreak of hantavirus pulmonary syndrome in the Four Corners area of the United States in 1993. The response to anomalous laboratory results turned out to be a critical factor during the investigations of Legionnaires' disease and Pontiac fever. Epidemiologic investigations quickly established that the disease was airborne, but there was no evidence of person-to-person spread. The pulmonary manifestations of the Philadelphia patients were considered more typical of viral pneumonia than of bacterial.